Title
HUMAN VISUAL PROCESSES INVOLVED IN STEREO AND ILLUSORY CONTOUR-DEFINED IMAGES ARE PREDOMINANTLY LOCATED IN RIGHT OCCIPITAL CORTEX USING FUNCTIONAL MAGNETIC RESONANCE IMAGING (fMRI).

Authors
J. Hirsch^1,3, R. DeLaPaz^2,3, J. Victor³, T. Li^*1, K. Kim^1,3,P. Borden¹, K.M. Lee^1,3, N. Rubin⁴, and R. Shapley⁵. Dept of Neurology¹ and Radiology², MSKCC; Depts of Neurology and Neuroscience, Cornell U. Medical College³, NY, NY 10021; Depts of Psychology, Harvard U.⁴, Cambridge, MA; Center for Neural Science, NYU⁵, NY, NY 10021.

Using fMRI to study intermediate levels of human visual representation, we previously discovered that cortical responses to illusory contours were primarily found in right occipital cortex. Our current goal is to determine the relationship between these illusory contour areas and areas that respond to stereo-defined contours. Images were acquired on a GE Signa 1.5T scanner equipped for echoplanar imaging with inplane resolution of 1.5 x 1.5mm2. Sixteen contiguous axial slices were acquired during baseline, visual stimulation, and post-stimulation periods. Signal intensities for each pixel were compared using a multistage statistical process to identify active regions. Four stimulation conditions were compared: 1) four "pacmen" at the corners of a Kanizsa square (illusory contours); 2) four non-aligned "pacmen" in the same locations (no illusory contours); 3) red-green random spot anaglyphs that produced a stereo-defined square; and 4) a random spot with no stereo. Activity elicited by illusory-contour-unique and stereo-contour-unique stimuli were identified by comparison of conditions 1 &2 and 3 & 4 respectively. The activity elicited by stereo-contours was observed primarily in right occipital cortex for all subjects and in discrete locations adjacent to illusory contour activity. Thus we find a cluster of adjacent areas in the right hemisphere activated by visual segmentation in at least two modalities.

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Support by MSKCC (JH, RD), C.V. Starr Foundation (NR), EY7977 (JV), Morris Foundation (KK), and EY1472 (RS).